Styrenic block copolymers of conjugated diene monomers and vinyl aromatic monomers have been widely used in the fields of footwear, plastic modifiers, asphalt modifiers, adhesives, and the like, and as packaging materials for home products, household appliances and industrial components, and the like, toys and the like because even when the styrenic block copolymers are not vulcanized, the copolymers have the same elasticity as vulcanized natural rubber and synthetic rubber at room temperature and also have the same excellent processability as thermoplastic resins at high temperatures.
Further, a hydrogenated product of the styrenic block copolymer is widely put in practical use as automobile parts, medical devices, and the like in addition to the application fields as described above because it is excellent in weather resistance and heat resistance.
In particular, since a hydrogenated styrenic copolymer including a conjugated diene moiety having a high vinyl bond amount has high compatibility with polypropylene, it is used as a modifier of polypropylene, and compositions thereof are used for various applications.
For example, Patent Document 1 discloses a composition of a hydrogenated block copolymer and a polypropylene resin, the hydrogenated block copolymer being prepared by hydrogenating a copolymer having a block of vinyl aromatic hydrocarbon and a block of a diene moiety having a vinyl bond amount of 62% or more.
However, in order to prepare a conjugated diene moiety having a high vinyl bond amount like the block copolymer described in Patent Document 1, the step of polymerization at a low temperature for a long time is generally required, which has a problem of low productivity.
In order to solve the problem of polymerization time for achieving such a high vinyl bond amount, Patent Document 2 proposes a method for producing an unhydrogenated copolymer in which a dialkyl ether and an alkali organic compound are used in a polymerization step and a molar ratio of the alkali organic compound to an organolithium compound is set to 0.5 mol or more.
Further, Patent Document 3 proposes a method for producing an unhydrogenated copolymer in which a polarity regulator, Na alkoxide, and an organolithium compound are used in a polymerization step and a molar ratio of the Na alkoxide to the polarity regulator is set in a range of from 0.1 to 10 and a molar ratio of the Na alkoxide to the organolithium compound is set in a range of from 0.1 to 10.
Furthermore, Patent Document 4 proposes a method for producing an unhydrogenated copolymer in which a polar agent, a metal salt, and an organolithium compound are used in a polymerization step and a molar ratio of the polar agent to the organolithium compound is set to 3 or more, a molar ratio of the metal salt to the organolithium compound in a range of from 0.01 to 2, and a molar ratio of the metal salt to the polar agent in a range of from 0.001 to 0.5.
On the other hand, improvement in the rate of hydrogenation is a major problem for obtaining a hydrogenated copolymer in terms of economical efficiency.
In view of such a problem, Patent Document 5 proposes a method for producing a hydrogenated copolymer in which a tertiary amine compound and sodium alkoxide are allowed to be present together in a polymerization step, thereby improving the rate of a subsequent hydrogenation step.